Troops loyal to Ivory Coast's president elect, Alassane Ouattara, in Duekoue city, where the Red Cross says 800 people have been killed. Photograph: Zoom Dosso/AFP/Getty Images

Rival forces in Ivory Coast are continuing their battle for power as it emerged that more than 800 people were killed this week in inter-ethnic violence in the town of Duekoue.

Soldiers backing the country's UN-recognised president, Alassane Ouattara, clashed with forces loyal to the voted-out president, Laurent Gbagbo, in the country's main city and former capital, Abidjan.

Gbagbo's military spokesman, Lieutenant Colonel Alphonse Guano, made a televised address after his forces retook the state TV headquarters on Saturday.

Guano called on security forces to report for duty to resist attacks by forces loyal to the internationally recognised president, Alassane Ouattara, whose fighters now control about 80% of the country.

Pro-Ouattara forces made a rapid advance through the country this week. The International Committee of the Red Cross has said at least 800 people were killed in intercommunal violence in Duekoue believed to have taken place on Tuesday, the day after the town in the west of Ivory Coast was taken by the pro-Ouatarra fighters.

It is not clear what prompted the killings and whether Ouattara's forces were involved.

UN deputy spokesman Farhan Haq said the UN High Commissioner for Human Rights had received "unconfirmed but worrying reports" that the pro-Ouattara force "has been committing human rights violations" during the advance toward Abidjan.

But the faction has denied taking any part in the attrocities and blamed any killings on the retreating Gbagbo fighters.

Red Cross spokeswoman Dorothea Krimitsas said delegates from the Ivorian Red Cross had visited Duekoue on Thursday and Friday to gather evidence and saw a "huge number of bodies".

"We have information that at least 800 persons were killed on 29 March in Duekoue in intercommunal violence," Krimitsas told Reuters. "Our colleagues saw hundreds of bodies. We strongly suspect that was the result of intercommunal violence. Since Monday or so tens of thousands of people have fled the area. This is not the first time there has been intercommunal violence in Duekoue."

The head of the ICRC delegation in the country, Dominique Liengme, said in a statement: "This incident is particularly shocking in its size and brutality.

"The ICRC condemns direct attacks on civilians and reminds the parties to the conflict to make sure that people in the territory under their control must be protected under all circumstances."

The ICRC said tens of thousands of women, men and children had fled fighting in Duekoue since Monday.

Gunfire and the sound of heavy weapons fire rang out across Abidjan as the country's former rebels pressed an offensive to oust Gbagbo, who is refusing to leave office.

Pro-Ouatarra fighters met with resistance from Gbagbo fighters around strategic locations like the presidential palace, the state broadcaster RTI and military bases.

Residents said they heard loud explosions near the Agban base, the city's largest, in the Adjame neighbourhood near Cocody where Gbagbo has his official residence.

"Mortar fire has been heard since late last night around the gendarmerie. It is very loud and we're taking shelter in our homes," said local resident Jules Konin.

"The gendarmes from the camp are fighting the insurgents," said another resident, Adi Saba.

Ouattara was internationally recognised as president last year after the electoral commission declared him the winner of a November run-off vote. But Gbagbo also claimed victory. Sanctions have failed to dislodge Gbagbo.

The four-month standoff since the election has killed nearly 500 people, according to UN figures, although the real toll is probably far higher. Around one million people have fled Abidjan alone and 122,000 more have crossed into Liberia, the UN says.